Total gaseous chlorine (TCl_g) measurements can improve our understanding of unknown sources of Cl to the atmosphere. Existing techniques for measuring TCl_g have been limited to offline analysis of extracted filters and do not provide suitable temporal information on fast atmospheric process. We describe high time-resolution in-situ measurements of TCl_g by combusting ambient air over a heated platinum (Pt) substrate coupled to a cavity ring-down spectrometer (CRDS). The method relies on the complete decomposition of TCl_g to release Cl atoms that react to form HCl, for which detection by CRDS has been shown to be fast and reliable. The method was validated using custom organochlorine permeation devices (PDs) that generated gas-phase dichloromethane (DCM), 1-chlorobutane (CB), and 1,3-dichloropropene (DCP). The optimal conversion temperature and residence time through the high-temperature furnace was 825 °C and 1.5 seconds, respectively. Complete conversion was indicated by the near unity orthogonal distance regression analysis slope (±σ) of 0.996 ± 0.012, 1.048 ± 0.006, and 1.027 ± 0.061 for DCM, CB, and DCP, respectively. Breaking these strong C-Cl bonds represents a proof of concept for complete conversion of all similar or weaker bonds that characterize all other TCl_g. We applied this technique to both outdoor and indoor environments and found reasonable comparisons in ambient background mixing ratios with the sum of expected HCl from known Cl species. We measured the converted TCl_g in an indoor environment during cleaning activities and observed varying levels of TCl_g comparable to previous studies. The method validated here is capable of measuring in-situ TCl_g and has a broad range of applications to make routine TCl_g measurements in a variety of applications.